Expose text from all slots to AbuseFilter
This is a first step towards MCR support in AbuseFilter. The textual
representation of all slots is concatenated. Since AbuseFilter uses
getTextForSearchIndex to determine the textual representation of
content, blind concatenation should not break any assumptions
made by AbsueFilter rules: this naive approach is no worse than
AbuseFilters handling of non-textual content in general, and should
work fine for textual content.